In many respects, the four stroke cycle gasoline engine and the four stroke cycle diesel engine are very similar. They both follow an operating cycle consisting of intake, compression, power, and exhaust strokes. They also share the same system for intake and exhaust valves.

The main differences between gasoline engines and diesel engines follow:

(1) In a diesel engine the fuel and air mixture is ignited by the heat generated by the compression stroke, versus the use of a spark ignition system in a gasoline engine. The diesel engine therefore needs no ignition system. For this reason, the gasoline engine is referred to as a spark ignition engine and a diesel engine is referred to as a compression ignition engine.

(2) In a diesel engine the fuel and air mixture is compressed to about onetwentieth of its original volume. In contrast, the fuel and air mixture in a gasoline engine is compressed to about one eighth of its original volume. The diesel engine must compress the mixture this tightly to generate enough heat to ignite the fuel and air mixture.

(3) The gasoline engine mixes the fuel and air before it reaches the combustion chamber. A diesel engine takes in only air through the intake port. Fuel is put into the combustion chamber directly through an injection system. The air and fuel then mix in the combustion chamber.

(4) The engine speed and the power output of a diesel engine are controlled by amount of air is constant. This contrasts with the gasoline engine where the speed and power output are regulated by limiting the air entering the engine.

Advantages.
(1) The diesel engine is much more efficient than a gasoline engine due to the much tighter compression of the fuel and air mixture. The diesel engine produces tremendous lowspeed power, and gets much greater fuel mileage than its gasoline counterpart. This makes the engine very suitable for large trucks.

(2) The diesel engine requires no ignition tuneups because there is no ignition system.

(3) Because diesel fuel is of an oily consistency and is less volatile than gasoline, it is not as likely to explode in a collision.

d. Disadvantages.
(1) The diesel engine must be made very heavy to have enough strength to withstand the tighter compression of the fuel and air mixture.

(2) The diesel engine is very noisy.

(3) Diesel fuel creates a large amount of fumes.

(4) Because diesel fuel is not very volatile, cold weather starting is difficult.

(5) A diesel engine operates well only in lowspeed ranges in relation to gasoline engines. This creates problems when using them in passenger cars, which require a wide speed range.

e. Usage. Diesel engines are widely used in all types of heavy trucks, trains, and boats. In recent years, more attention has been focused on using diesels in passenger cars.

f. Multifuel Engine. The multifuel engine is basically a four stroke cycle diesel engine with the capability of operating on a wide variety of fuel oils without adjustment or modification. The fuel injection system is equipped with a device called a fuel density compensator.

Its job is to vary the amount of fuel, keeping the power output constant regardless of the fuel being used. The multifuel engine uses a spherical combustion chamber to aid in thorough mixing, complete combustion, and minimized knocks.